Investment casting

ABSTRACT

Methods are provided for producing investment cast articles, such as orthopedic implants or portions thereof, having at least a partially textured surface that is formed during casting of the article. In an exemplary method, a textured metal casting is produced by creating a heat destructible pattern and spraying the pattern with a texturing material to cause the texturing material to form a textured surface on at least a portion of the pattern. In another embodiment, a textured template is pressed against a heat softenable pattern to provide a textured pattern. With respect to each of these methods, a shell is the created around the textured pattern form a mold, and the pattern is removed from the shell. Molten metal is introduced into the mold and allowed to harden, after which the mold is removed. In yet another embodiment a textured pattern for investment casting is provided by creating a textured model and enveloping it with a resilient material to create a resilient mold. The textured model is removed from the resilient mold and the mold is filled with investment casting wax to create a textured pattern.

FIELD OF THE INVENTION

The present invention relates to investment casting, and moreparticularly to the manufacture of investment cast articles, such asorthopedic implants, with a textured surface.

BACKGROUND OF THE INVENTION

Various metal casting processes, such as investment (or "lost wax")casting are well known for the fabrication of metal objects. Thisprocess requires several steps, the first of which is to create orprovide a pattern or shape to be replicated. The pattern, often made ofwax, is used to make a mold that is then used to form cast metalarticles.

Typically, several wax patterns are joined together on a wax "tree" toenable the simultaneous manufacture of several parts. The tree is asolid wax tube that has side walls to which a stem of each wax patternis joined to form a cluster. The wax tree defines what will become agate leading to passages for allowing molten metal to travel through themold to each cluster and part pattern. Once all of the wax patterns arejoined to the wax tree, the cluster is coated with one or more coats ofa refractory by dipping the wax pattern-tree assembly in a ceramicslurry. After the slurry dries, fabrication of a shell or mold iscompleted by heating the slurry coated wax to cure or harden theceramic, and to melt out the solid wax patterns and the wax tree. Moltenmetal is then poured into the shell so that it fills each of thecavities formerly occupied by the wax patterns and the wax tree. Afterthe metal has cooled and hardened, the shell is fractured and removed,and the cast metal parts are severed from the metal tree. The cast partsare then subjected to post-machining, grinding off the gates, beadblasting, and polishing, as required.

With respect to medical implants, such as joint prosthesis components,it has been discovered that texturing or roughening the surface of acast metal implant can improve the interface and fixation between theimplant and the bone, with or without bone cement. The creation of aroughened surface on an implant, whether it was investment cast orforged, is typically one of the last steps in the manufacturing processof the component. Known methods for obtaining a roughened surfaceinclude grit-blasting, grinding, direct machining, laser etching, andsintering of beads to the surface of the implant.

Known surface texturing techniques, however, have severe short-comingswith respect to manufacturing speed, efficiency, and cost, as well asstructural limitations and deficiencies. For example, if the surface ofthe implant is improperly roughened, the near finished implant must bediscarded. Depending on the alloy used to fabricate the implant and thesize of the implant, the creation of unacceptable texturing cansignificantly increase manufacturing cost. The creation of a texturedsurface requires a skilled craftsperson. Therefore, for other thansimple, uniform texturing, precise replication of a particularconfiguration is difficult to achieve ink even limited productionquantities. Furthermore, known surface texturing techniques areincapable of creating certain complex shapes and patterns, such asundercutting, which foster bone ingrowth and more secure fixation. Yetanother disadvantage of known techniques for texturing a previously-castimplant is that the application of texturing materials, such as with aplasma-spray process results in an imperfect bond between the sprayed-onmaterial and the implant which can lead to abrasive detachment and weaksubstrate coating interfaces. Similarly, sintering beads on a surfacecauses loss of favorable mechanical properties of the implant.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of known surfacetexturing techniques by providing a heat destructible pattern having atextured surface. The textured pattern is used to create an investmentcasting mold which in turn is used to create the cast article. Thistechnique allows very specific and/or complex patterns to be integrallyformed with the implant surface as cast.

In an exemplary method, a textured metal casting, such as an orthopedicimplant or a component thereof, is produced by creating a heatdestructible pattern and spraying the pattern with a texturing materialto cause the texturing material to form a textured surface on at least aportion of the pattern. The texturing material can include molten waxdroplets that adhere to and/or melt at least a portion of the surface ofthe pattern or solid particles that adhere to and/or indent the surfaceof the pattern. The texturing material can be evenly or unevenly sprayedagainst the pattern at a selected temperature and velocity to create adesired texture. A shell is created around the textured pattern to forma mold, and the pattern is removed from the mold. Molten metal isintroduced into the mold and allowed to harden, after which the mold isremoved from the textured casting.

In another embodiment, a textured template is pressed against a heatsoftenable pattern to provide a textured pattern. The textured templatecan be formed by creating a data file that defines a three-dimensionaltextured template; providing the data file to a rapid prototypingmachine; and forming a model representative of the three-dimensionaltextured template with the rapid prototyping machine. A second texturedtemplate can be provided that has a different texture than the firsttextured template.

In yet another embodiment, a textured pattern for investment casting isprovided by creating a textured model and enveloping it with a resilientmaterial, such as silicone, to create a resilient mold. The texturedmodel is removed from the resilient mold and the mold is filled withinvestment casting wax to create a textured pattern The textured modelcan be fabricated with a rapid prototyping machine that performsstereolithographic operations, which enable the model to include verycomplex geometries, as well as undercuts, that are particularly wellsuited for bone ingrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and the attendantadvantages and features thereof will be more readily understood byreference to the following detailed description when it is considered inconjunction with the accompanying drawings wherein:

FIG. 1 is a simplified illustration of a pattern being sprayed with atexturing material;

FIG. 2 is a sectional view of a portion of a pattern having wax spraytexturing to create positive relief features;

FIG. 3 is a sectional view of a portion of a pattern having wax spraytexturing to create negative relief features;

FIG. 4 is a sectional view of a portion of a pattern having solidparticle texturing;

FIG. 5 is a schematic view of a pattern being textured with a template;

FIG. 6 is a schematic illustration of the creation of a model using acomputer-assisted design system;

F1G. 7 is a schematic illustration of a rapid prototyping machine makingan object corresponding to a computer-assisted design model of FIG. 6;

FIG. 8 is an illustration of the object of FIG. 7 encased in a resilientmaterial; and

FIG. 9 is a perspective view of one-half of a soft mold.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of a method of making a textured investment castingbegins with the step of creating a custom heat destructible pattern orselecting an existing pattern from stock supplies. Typically, patternsare made of investment casting wax, however, many plastics are alsoacceptable, as are hybrid wax/plastic patterns. As used herein, a"pattern" is full-scale representation or model of any article that iseither machine or hand made. Exemplary patterns include components fororthopedic implants and portions thereof.

After the pattern is selected or created, a heat destructible texturingmaterial is selected for spraying against the pattern. Exemplarytexturing materials include wax droplets or beads that are molten, soft,or hard, and plastic particles or beads. However, any material which maybe applied to the pattern and which will subsequently burn out of a moldcleanly may be used. As used herein, "particle" or "droplet" is intendedto encompass any size or shape configuration obtainable with a giventexturing material, and the particular size and configuration ormorphology of a selected particle or droplet is selected to achieve adesired texture.

Following selection of a texturing material, a spray is created with thetexturing material and it is directed toward the pattern or the patternis placed in the path of the spray. As used herein, a "spray" isintended to encompass everything from a concentrated, narrow stream to adiffuse mist, and "spray" should be broadly construed to mean placingthe texturing material or a portion thereof in motion. The particulardispersion, concentration, temperature, and pressure of the spray aredetermined by the texturing material and the desired texture for thepattern. As illustrated in FIG. 1 molten wax or solid particles 10, suchas wax or plastic beads under pressure can be ejected from a nozzle 12to create a spray. A pattern 14 is shown in the path of the spray.

FIG. 2 illustrates a pattern 14 that has been sprayed by molten waxdroplets, wherein the molten wax temperature and spray velocity weresuch that at least some of the molten wax droplets adhered to at least aportion of the surface 16 of the pattern to create surface texturing,roughness, or bas relief. IN FIG. 2, the droplets have been deposited toform bumps 18 that can be defined as positive surface relief features,on a portion of the pattern surface 16.

FIG. 3 illustrates a pattern 14 that has been sprayed by molten waxdroplets, wherein the molten wax temperature and spray velocity weresuch that at least some of the molten wax droplets melted away at leasta portion of the surface 16 of the pattern to create surface texturing,roughness, or bas relief. In FIG. 3, the droplets have been createddepressions, voids, or indentations 20 that can be defined as negativesurface relief features, on a portion of the pattern surface 16.

The temperature of the spray of molten wax can be regulated to createpositive and negative surface features in the same surface areas. Also,the spray of texturing material can be directed toward the pattern 16,or a portion thereof, to evenly or unevenly distribute the spray tocreate, respectively, a substantially uniform or irregular pattern.Furthermore, although the bumps 18 and indentions 20 appear rounded inthe illustrations, their shapes can range from circular to linear, andthe bumps and indentations can have angled or convoluted boundaries.

FIG. 4 illustrates a pattern 22 that has been sprayed by solid particles24, wherein the particle temperature, spray velocity, and particlemorphology were such that at least some of the particles adhered to atleast a portion of the surface 26 of the pattern 22 to create surfacetexturing, roughness, or bas relief. The particles 24 form bumps 28 thatcan be defined as positive surface relief features, on a portion of thepattern surface 26. The particles can range from being barely to fullyimpacted in the surface of the pattern. Bumps are also created by thedisplacement of pattern material in response to particle impact.

FIG. 4 also illustrates additional surface texturing, roughness, or basrelief in the form of depressions, voids, or indentations 30 that can bedefied as negative surface relief features. These indentations arecreated by impact of the particles 24 with the surface 26 and thenbouncing off or not adhering to the surface. As with respect to themolten wax spray technique, the temperature of the particulate spray canbe regulated to create positive and negative surface features in thesame surface areas. Similarly, the particulate spray of texturingmaterial can be directed toward the pattern 22, or a portion thereof, toevenly or unevenly distribute the spray to create, respectively, asubstantially uniform or irregular pattern. Furthermore, although thebumps 28 and indentions 30 appear rounded in the illustrations, theirshapes can range from circular to linear, and have angled or convolutedboundaries. If the particles impact the surface at an angle, anunder-cut surface 32 is created.

Yet another technique for creating a textured pattern includes pressinga textured template 34 against at least a portion of a heat softenablepattern 36 to provide a textured pattern, as shown in FIG. 5. Either thetextured template 34, the pattern 36, or both can be heated tofacilitate pattern creation. Alternatively, the template can create atexture on the pattern by pressing the template against the pattern withenough pressure to deform the pattern. This technique is well suited forapplications requiring a precise and/or complicated pattern geometry,and it is particularly well suited for providing a texture that promotesbone ingrowth.

In an exemplary embodiment, the template 34 is a metal object fabricatedusing any known technique for creating a metal object. Alternatively,the template 34 can be fabricated from a plastic that has a highermelting/distortion point than the wax pattern. However, in otherembodiments, the template is created with a rapid prototyping machine.The object created by the rapid prototyping machine can itself be usedas the textured template, or it can be used as a model for the creationof an investment casting mold.

For example, a textured template can be provided by creating a data filethat defines a three-dimensional textured template, wherein the texturedtemplate includes a texture pattern configured to promote bone ingrowth.The data file is provided to a rapid prototyping machine capable ofcreating a heat destructible model or a ceramic model. A heatdestructible model representative of the three-dimensional texturedtemplate is then created with the rapid prototyping machine. A heatresistant shell is formed around the heat destructible model and heat isapplied to the shell to remove the heat destructible model therefrom.Molten metal is poured into the shell; the molten metal hardens; and theshell is removed. The thus formed textured template is heat pressedagainst at least a portion of the pattern to provide a textured pattern.A second textured template having a different texture than the firsttextured template can be provided and pressed on at least a portion ofthe pattern to provide a pattern with different textures on differentregions thereof.

Each of the above techniques provides different manufacturingadvantages. However, should a problem be encountered in the texturingsteps, the cost associated with scrap is greatly reduced as onlyinexpensive wax is sacrificed rather than a nearly finished product.Accidentally roughened surfaces of a wax pattern are easily smoothed byheating the wax.

Having been imparted with a surface texture, the pattern, created bywhatever method, is used to create a mold. En an exemplary method, thepattern is coated with particles of a refractory material by spraying ordipping to create a shell around the textured pattern. The texturedpattern is removed from the shell by the application of heat whichcauses the pattern, and any blast media impacted thereon, to melt, burn,or vaporize, allowing it to be drained or exhausted from an opening inthe shell. Heating, either in this or a subsequent step, cures therefractory material to complete the mold making process.

Subsequent casting steps are not discussed in great detail, as they arewell known to those skilled in the art of metal casting. These stepsinclude introducing molten metal, such as Cr--Co--Mo alloy (ASTM F75)through one or more gates in the mold. The mold and the metal areallowed to cool, and the mold is removed from the hardened metal. Thetextured implant can then be subjected to post-machining, grinding offof gates, bead blasting, and polishing, as required.

Yet another embodiment of the invention is illustrated by FIGS. 6-9,wherein a textured pattern for investment casting is provided bycreating a textured model and enveloping it with a resilient material tocreate a resilient mold. The method begins by creating a textured model,and although the textured model can be created by any of the techniquesdiscussed above, a model created with a rapid prototyping machine isparticularly advantageous as it permits fabrication of configurationsand textures not obtainable by conventional methods.

Referring now to FIG. 6, an input device 40, such as a keyboard, mouse,light pen, and the like, is associated with a computer 42 and a displayunit 44, to facilitate the creation of a data file that defines athree-dimensional article 46 representative of at least a portion of abioimplantible article. The data file can be created by any of a numberof drawing, graphic, aided design program (CAD) is "Pro Engineer,"available from Parametric Technology design, or solid modeling programsknown to those skilled in the art. An exemplary computer Corporation.The data file can also define bleeders and gates used in casting asrequired.

By initially creating the article 46 as a CAD model, the article can bereviewed "on screen" or as computer generated drawings by asurgeon/engineer and modified as required prior to actually creating amold or an implant. To further improve the review and design process, athree-dimensional model can be fabricated to assess potentialmodifications. Furthermore, the complexity and subtlety of the model isgreatly enhanced. Special contoured surfaces, undercuts, internal cores,or typically machined features, as well as other features that are notmachineable, can be easily incorporated into a CAD model and built witha rapid prototyping machine due to its layered fabrication capability.

In a subsequent step, a data file 48 that has been created as describedabove is provided to, or accessed by, a rapid prototyping machine 50capable of transforming the data into a tangible object 52. Exemplaryrapid prototyping machines include: selective laser sintering, solidground curing, fused deposition modeling, Stereo-Lithography, LOM, andthree-dimensional printing.

In an exemplary embodiment of the method, the data defines abioimplantible article having a surface configuration that promotes boneingrowth. For example, the article can include several discrete layerseach having a different configuration and/or porosity. These layers canhave a 10% to 20% porosity difference from layer to layer. Also, thearticle can include different regions, wherein the porosity is differentin each region. Although there are many ways to configure the surface tomake it porous, an exemplary article is defined to include generallyround or oval pores that provide openings in the surface of the article,such as an orthopedic implant. The pores are in the range of 50 to 400microns in diameter and have a depth in the range of 175 to 1000microns; and the implant surface can have an open porosity in the rangeof 30% to 70% of its total surface area.

After the textured article or model 52 has been fabricated, by whatevermethod, a resilient material 54, such as room temperature vulcanization(RPV) silicone is applied to the model to envelop it, thus creating aresilient mold of the textured model as shown in FIG. 8. Prior toenvelopment, one or more dividers 56 or partitions can be placed aroundthe model to create one or more parting lines in the mold in a mannerknown to those skilled in the art, which allows the mold to be separatedinto two or more portions.

Once the portions of the resilient material 54 are separated, thetextured model 52 is removed from the resilient material. As shown inFIG. 9, the resilient material 54, defining half of a mold, can be athin layer inlaid in a support structure 56 which reinforces theresilient material around and behind the cavity 58 defmed by the mold.In other embodiments, the resilient material 54 has a thicknesssufficient to provide adequate stability for molding without requiring asupplemental support structure.

The mold is filled with a modeling material such as investment castingwax, using techniques known to those skilled in the art. When cured orhard, the textured wax pattern can be removed from the resilient mold byflexing the mold or peeling it away from the wax model, depending on thethickness and properties of the resilient material. The flexible moldcan then be filled wit wax again and again to replicate the model.Additional molds are readily fabricated using the original rapidprototype model. The textured wax models or patterns are subsequentlysubjected to known investment casting operations to make textured metalcastings.

Yet another way to impart surface texturing to the wax model, instead ofor in addition to the above described techniques, is to place one ormore forms 60 having a textured face onto the surface of the resilientmaterial 54 that defies a mold, textured face down toward the surface ofthe resilient material. When the mold is filled with wax, the texturedform 60 or forms bond with the wax and thereby define a portion of theouter surface of the wax model. This technique is particularly wellsuited for creating a model with undercuts, cores, and other complexconfigurations. In an exemplary embodiment, the form 60 are forms arethin ceramic plaques having a selected pattern or texture.

Although the invention has been shown and described with respect toexemplary embodiments thereof, various other changes, omissions andadditions in form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method of making a textured pattern forinvestment casting, comprising the steps of:creating a textured modelwherein a data file defines a three-dimensional shape of at least aportion of an orthopedic implant, and wherein the three-dimensionalshape includes a plurality of layers each having a different porosity;providing the data file to a rapid prototyping machine; creating atextured model representative of the data file with the rapidprototyping machine; enveloping the textured model representative of thedata file with a resilient material to create a resilient mold of thetextured model; removing the textured model representative of the datafile from the resilient mold; filling the resilient mold with investmentcasting wax to create a textured pattern; and removing the texturedpattern from the resilient mold.
 2. The method of claim 1, wherein thethree-dimensional shape is configured to promote bone ingrowth.
 3. Themethod of claim 1, wherein the step of creating a data file includes thestep of defining pores in at least a portion of the orthopedic implant,wherein the pores define openings in the surface of the implant that arein the range of 50 to 400 microns in diameter and wherein the pores havea depth in the range of 175 to 1000 microns.
 4. The method of claim 3,wherein the step of creating data file further includes the step ofdefining an implant surface having an open porosity in the range of 30%to 70%.
 5. The method of claim 1, wherein the difference in porosityranges from 10 to 20 percent.
 6. A method of making a textured patternfor investment casting, comprising the steps of:creating a texturedmodel wherein a data file defines a three-dimensional shape of at leasta portion of an orthopedic implant, and wherein the three-dimensionalshape includes a plurality of surface regions each having a differentporosity; providing the data file to a rapid prototyping machine;creating a textured model representative of the data file with the rapidprototyping machine; enveloping the textured model representative of thedata file with a resilient material to create a resilient mold of thetextured model; removing the textured model representative of the datafile from the resilient mold; filling the resilient mold with investmentcasting wax to create a textured pattern; and removing the texturedpattern from the resilient mold.
 7. The method of claim 6, wherein thethree-dimensional shape is configured to promote bone ingrowth.
 8. Themethod of claim 6, wherein the step of creating a data file includes thestep of defining pores in at least a portion of the orthopedic implant,wherein the pores define openings in the surface of the implant that arein the range of 50 to 400 microns in diameter and wherein the pores havea depth in the range of 175 to 1000 microns.
 9. The method of claim 8,wherein the step of creating a data file further includes the step ofdefining an implant surface having an open porosity in the range of 30%to 70%.
 10. A method of making a textured pattern for investmentcasting, comprising the steps of:creating a textured model wherein adata file defines a three-dimensional shape of at least a portion of anorthopedic implant, and wherein the three-dimensional shape includes aplurality of layers each having a different configuration; providing thedata file to a rapid prototyping machine; creating a textured modelrepresentative of the data file with the rapid prototyping machine;enveloping the textured model representative of the data file with aresilient material to create a resilient mold of the textured model;removing the textured model representative of the data file from theresilient mold; filling the resilient mold with investment casting waxto create a textured pattern; and removing the textured pattern from theresilient mold.
 11. The method of claim 10, wherein thethree-dimensional shape is configured to promote bone ingrowth.
 12. Themethod of claim 10, wherein the step of creating a data file includesthe step of defining pores in at least a portion of the orthopedicimplant, wherein the pores define openings in the surface of the implantthat are in the range of 50 to 400 microns in diameter and wherein thepores have a depth in the range of 175 to 1000 microns.
 13. The methodof claim 12, wherein the step of creating a data file further includesthe step of defining an implant surface having an open porosity in therange of 30% to 70%.